To find diagonal $ d1 $ use formula:

$$ \sin \left( \frac{ \alpha }{ 2 } \right) = \dfrac{ d_2 }{ d_1 } $$

After substituting $\alpha = 120^o$ and $d_2 = 30\, \text{cm}$ we have:

$$ \sin \left( \frac{ 120^o }{ 2 } \right) = \dfrac{ d_2 }{ d_1 } $$ $$ \sin( 60^o ) = \dfrac{ 30\, \text{cm} }{ d_1 } $$ $$ \frac{\sqrt{ 3 }}{ 2 } = \dfrac{ 30\, \text{cm} }{ d_1 } $$ $$ d_1 = \dfrac{ 30\, \text{cm} }{ \frac{\sqrt{ 3 }}{ 2 } } $$ $$ d_1 = 20 \sqrt{ 3 }\, \text{cm} $$

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